SPECT (Single Photon Emission Computerized Tomography) is used to study the three dimensional distribution of a radionuclide in a patient. Typically one or more radiopharmaceuticals are ingested or are injected into the patient. When radiopharmaceuticals are injected it is usually into the patient's blood stream, to image the cardio-vascular system or to image specific organs which absorb the injected radiopharmaceuticals. One or more gamma or scintillation detectors are positioned near the patient to record emitted radiation.
SPECT images are generally produced by:                (a) rotating the detector(s) around the patient in order to record emissions from a plurality of directions; and        (b) transforming the recorded emissions, using methods well known in the art, into a tomographical multi-slice image, a three dimensional image or some other representation of the distribution of the radiopharmaceutical injected into the patient's body.        
One problem with SPECT is that the tissues surrounding the organs being imaged attenuate and scatter the radiation emitted by the radiopharmaceutical, distorting the resulting SPECT images. To solve this problem, a SPTCT (Single Photon Transmission Computerized Tomography) image of the region being imaged, is acquired, simultaneously with the SPECT image. The SPTCT image provides information regarding the attenuation and scattering characteristics of the region being imaged, so that the multi-view emission data can be corrected.
In order to acquire the simultaneous SPTCT image, a source of radiation is placed opposite the patient's body from the detectors(s) and rotated with the detector(s). Preferably, but not necessarily, the energy of the SPTCT source is different from that of the radiopharmaceutical so that the detector is able to easily differentiate the two radiations.
Since the emission image is acquired at the same time as the transmission image, and the relative geometry of the SPTCT and SPECT systems are known, the images are easily registered to one another.
The diagnostic method that uses SPECT and SPTCT simultaneously is known as STET (Simultaneous Transmission and Emission Tomography). This method is described in further detail in U.S. Pat. No. 5,210,421, the disclosure of which is incorporated herein by reference.
One aspect of the present invention relates to the use of STET imaging techniques for functional imaging. In this use, the resultant STET image shows the metabolic activity of body tissue, since dead or damaged body tissue absorbs the radiopharmaceutical at a different rate (or not at all) from healthy tissue. When used in this manner, the STET image shows the functional activity of the body tissue, not its structural detail.
However, STET images have two drawbacks. First, as indicated above, the STET image does not show much structural detail; therefore, it is difficult to pinpoint where the imaged function is occurring in the patient's body. Many diagnostic imaging methods, in modalities other than nuclear medicine, reveal almost exclusively structure and not function, therefore, it is hard to compare STET images with other types of diagnostic images. Second, a common methodology, especially in cardiac examination, is to acquire a STET image shortly after injection of the radio pharmaceutical and to acquire another STET image of the same region after a certain period of time. By comparing these two (or more) images, it is possible to learn still more about the function of the tissue studied, such as the speed at which different portions of tissue absorb aid metabolize the radiopharmaceutical. However, if the two STET images are too different, it is not possible to closely compare them because the operator can not match the different parts of the images to each other.